While discussed here are the four industries, manufacturing, dairy, harvesting (agriculture) and fruit growing, shown in the twelve stamp US Parcel Post series, with the remaining eight grouped four and four as postal workers and postal transportation, I guess I need to point out the big picture. Parcel Post itself was and remains a major industry under a wholly different term: MAIL ORDER.
The US Department of Agriculture (USDA) produced guides for decades of which I own many but I am still looking for the one shown. Likewise new updates of what was available to local customers were posted in libraries and carried as new stories. I do own the the photo just due to my low energy due to my covid (see fifth post on this page, at Edit for perspective
at bottom http://goscf.com/t/81288&whichpage=2
) I am not bothering to re-photo it without the seller's watermark.
Parcel Post postage costs were set by zones based upon distance. Many new mail order firms located to central locations in the USA to minimize the cost to customers when ordering. For example many mail order cut glass companies located around the greater St. Louis area with examples of their mail order designs catalogs of the 1913-era still existing. Additionally other specialty companies arose, such as the companies supplying farmers packaging supplies for shipping farm goods, usually all created from cardboard.
In summary, in the USA, Parcel Post expanded mail order and caused a synergistic influence on many industries, if not an actual gestalt transformation of mail order itself.